1. Field of the Invention
The present invention relates to a main power supply using electric power supplied from an external power supply, a power supply device including an auxiliary power supply using electric power of an internal storage unit, and a control method for the power supply device.
2. Description of the Related Art
A copier, a printer, and a facsimile that utilize an electronic photographing process, and an image formation device such as a multifunction printer in which these are combined have come to be multi-functionalized recently, and the structures thereof have therefore become complicated. As a result, the maximum consumption power has been increasing.
The electric power supplied to a fixing heater has been increasing for reducing the user standby time or because of the factors of the image formation device itself such as the standby time till the startup of a fixing device or a suspension of an operation of printing or copying due to decrease in fixing temperature.
Meanwhile, there is an upper limit on the electric power that a commercial power supply as an external power supply can provide; therefore, when only a main power supply for supplying electric power from a commercial power supply is used, the supply power exceeds the range of the maximum consumption power. As a result, in an image formation device, an auxiliary power supply using an internal power storage unit is utilized and the electric power is supplied to a system of the image formation device by switching these two power supplies (for example, see Japanese Patent Application Laid-open No. 2004-236492).
In the invention described in Japanese Patent Application Laid-open No. 2004-236492, an electric power output circuit of a power storage unit, i.e., a power feeding circuit for a load is formed using a constant-voltage power supply; therefore, in the case where the output of an AC/DC power supply (main power supply) as a constant-voltage power supply and the output of an auxiliary power supply as a constant-voltage power supply similarly are fed to the load while they are switched, the voltage fluctuation occurs at the switching time due to the difference in output voltage between the two constant-voltage power supplies.
The occurrence of voltage fluctuation causes the operation of a motor that receives the electric power to be unstable, resulting in the problem of stop of the motor or uneven rotation of the motor. The uneven rotation of the motor leads to abnormality in image of an image formation device. For example, in the case of a color image formation device, color shift occurs.
As a power supply device for this problem, a power supply device is provided which includes: a main power supply for feeding electric power, which is supplied from an external power supply, to a load with the constant-voltage output; a load current detection unit for detecting a value of current flowing through the load; a control signal generation unit for generating a control signal based on the value of the current flowing through the load and an upper-limit instruction value for an output current value of the main power supply, the control signal making up for the current that exceeds the upper-limit instruction value; and an auxiliary power supply for feeding electric power of an internal power storage unit to the load with the constant-current output in accordance with the control signal (for example, see Japanese Patent Application Laid-open No. 2007-236159).
The power supply device according to Japanese Patent Application Laid-open No. 2007-236159 can simultaneously supply both the electric power from the main power supply and the electric power from the auxiliary power supply to the load, thereby eliminating the power feeding switching from one to the other and eliminating the voltage fluctuation that is caused by the switching.
However, the power supply device according to Japanese Patent Application Laid-open No. 2007-236159 described above has had a problem in that the power feeding from the auxiliary power supply stops when the power feeding from the external power supply has stopped due to a power cut or the like because the control signal generation unit cannot generate the control signal.
There is a need to make it possible to supply power to the load even if power supply from the external power supply is stopped, in a power supply device including a main power supply that feeds power to the load with constant-voltage output using the external power supply as an electric power supply source, and an auxiliary power supply that feeds power to the load with constant-current output using an internal power storage unit as an electric power supply source, wherein a part of load current by which part the load current exceeds an upper-limit instruction value of an output current value of the main power supply is fed from the auxiliary power supply.